Okay...it's a month later and the car still isn't driveable. As pointed out by wmburns, I ruled out other possible external sources of a vacuum leak (brake booster, EGR, EVP, etc) and decided to just do some "exploratory surgery". Replaced intake gaskets and also inspected and replaced the timing chain. The original timing chain exhibited deflection very close to the specified limit.
Now that it's reassembled, the engine starts fine (cold start) and comes up on fast idle for a bit before dropping to idle rpm, just as it always had. After running okay for a bit (15 seconds or so) at normal idle it will start skipping just a little and gets worse and worse until it either just shuts off or gets so rough it just can't maintain rotation. Fuel pressure seems pretty constant between 38-40psi and still am not getting a check engine light.
Just for kicks (and since I could) I borrowed a crank position sensor and gave that a shot. Had nothing to lose but obviously it didn't help.
I obtained a Mac tools MT97 scanner that I've attempted to use but I haven't been able to get any info out of it (supposed to be able to read data streams with it). The vast majority of the fields on the screens say "not supported". Figures.
At this point, unless a brainstorm appears in the very near future, I'm going to have to pay someone to tow the car to a shop (already paid once to get the car home) and then pay someone else to fix something that I could do myself if I only knew what to fix!! I find it quite frustrating but around here automotive techs don't make service calls, so getting one to come to my garage and do the diagnosis so I can make the repair is out of the question. I work at an agri/industrial dealer and we make service calls all the time, but comparing the two industries is a subject for another thread.
If anyone has any final thoughts, directions for use of MT97 scanner, or anything else that might be considered useful information or queries for further information, please feel free to post up!
I thought I wanted a career....turns out I just wanted a paycheck!
'70 Mach I
'95 Mustang coupe
'79 Mustang Mini-Stock